The present invention relates to a cathode for lithium secondary batteries, and more particularly to a cathode for lithium secondary batteries having a coating of active materials with improved properties
Lithium metal oxides (LiMO2) currently used as cathode active materials in lithium secondary batteries are typically in the form of spherical or pseudo-spherical powders. It can be advantageous to use a powder having superior adhesion between particles to improve electronic conductivity of the battery.
Polyvinylidene fluoride (PVdF) is a useful binder for lithium metal oxide particles. PVDF includes fluorine (F) atoms, which have the highest electronegativity of all of the elements of the periodic table, and hydrogen (H) atoms, which have the lowest electronegativity of all the elements of the periodic table. Accordingly, PVDF polymer includes a monomer with a molecular structure having a high dipole moment.
PVdF can be used as a binder for pole plates in lithium ion batteries and lithium polymer batteries, and typically has a number average molecular weight of from 130,000 to 220,000. PVDF commonly exists in α- and β-phases during its preparation. However, the α-phase is transformed into a distorted γ-phase during solvent casting of PVdF.
Generally, PVDF binder can be applied to a cathode by dissolving the binder in a suitable solvent, such as N-methylpyrrolidone (NMP), to form a solution, adding an active material to the solution, and mixing. Thereafter, a conductive agent can be added to the mixture and uniformly distributed to prepare a slurry. The slurry is coated to a uniform thickness on a collector, and dried to produce a cathode with coating solids formed on the collector.
As the liquid binder dries, the slurry changes to a solid state. That is, the binder exists in a solid state between the particles or between the collector and the particles to provide adhesion to the cathode. At this time, the PVdF is transformed into a β- or γ-phase. Since the PVDF has a structure wherein the constituent fluorine atoms are arranged in one direction, the dipole moment is greatly increased to induce the formation of a number of hydrogen bonds.
Due to this high polarity, hydrogen ions are highly susceptible to cations present in the solvent. When alkali ions, such as Li+ ions, of the cathode active material approach the hydrogen ions, the polar hydrogen ions bond to the fluorine to form hydrofluoric acid (HF), which is then deintercalated. Carbon atoms losing the ions share electrons to form carbon-carbon double bonds.
The double bonds thus formed are crosslinked by the presence of oxygen, water and other crosslinking-promoting compounds, resulting in gelation of the slurry. It can be difficult, however, to coat the gelled slurry uniformly on the collector, and in addition, the particles can adhere poorly to one another or poorly to the collector plate.
Poor adhesion between the particles can result in peeling of the particles from the surface of the cathode, leading to deterioration in the safety of the final batteries. That is, the peeled cathode particles may generate microshorts inside the batteries, thus deteriorating the performance of the batteries. Further, a number of microshorts may increase the risk of a fire due to shorting.
In addition, poor adhesion between the particles and the collector causes resistance to the transfer of electrons from the particles to the collector, thus reducing the electronic conductivity. As a result, high-rate characteristics and cycle characteristics may deteriorate.
After the slurry is coated onto the collector, the particles, typically applied to a thickness of hundreds of micrometers (μm) to the collector, undergo a pressing process. The particles can continuously stick to a rotating roll during pressing, which can result in poor surface quality of the cathode. In addition, too much pressure may be applied, which can form defects in the pole plate. Accordingly, poor adhesion between the particles and the collector can lead to low yield in the fabrication of batteries.